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Removing gray scale on hot-rolled steel [India] 

October 15, 2009

I am an entrepreneur in the field of Surface coating of Steel. My plant is located in India. Of late I am getting large sheet-metal parts having sheet thickness of less than 1 mm for coating.
Shot-blasting to removes gray oxide scales from surface is ruled out as it distorts the component. I am forced to do HCl acid pickling to remove the scales. But using HCl has seriously corroded the plant structures and equipment around. Besides, work-atmosphere is hazardous.
Is there any other cheap method to get rid of the gray scales? What are the methods used by large manufacturing companies to remove mill-scale?

Mahesh Behra
Technocrat and entrepreneur, coating plant - Pune, India

There are alkaline rust removers and electrolytic alkaline rust removers. You need to do some testing before you commit to anything as some scale requires a permanganate prep step which I find to be less than wonderful to use and to maintain.

You could try a lower strength sulfuric acid and you could try it electrolytically. There are blends of acids that are proprietary (more expensive) that work.

Finally, the blast will work if you lower the pressure and use one of the carbide grits. You need to do each side equally which will nearly eliminate the warping.

James Watts
- FL

Mahesh, 10-15 percent sulfuric acid at any temperature over about 55 degrees C works fine. Sulfuric acid does not release free acid into the air like hydrochloric does. It will actually work at any temperature down to ambient, but the lower the temperature, the slower the cleaning. We find that 55-60 degrees C is fine - at that temperature, it removes mill scale from hot-rolled structural sections in around 15 minutes which suits the operation of the shop.

Whichever acid you use, you must add an inhibitor to it, which allows the acid to attack the scale but severely retards attack on the base metal itself.

Bill Reynolds    consultant metallurgist Ballarat, Victoria, Australia

Ya Bill, sulfuric acid is a good option compared to HCl, but at elevated temperature it also creates fumes in the atmosphere. Besides, is it not more harmful to the Human skin?

Use of Inhibitors I guess will work well.

I learned that some branded companies sell pickling acid in a particular combination of both Sulfuric acid and HCl. Is it true?

Mahesh Behra
Surface coating - Pune, India

Sir:
You need a copy of my article on the Selection and Use of Pickling Acid Inhibitors which is published in the journal METAL FINISHING. The proper inhibitor reduces acid fumes by 90% to 99%. Obviously you need to know the proper concentration and temperature to use either acid (HCl or H2SO4). I have experimented with mixtures of HCl and H2SO4 and did not like the results. It is true that HCl fumes fill up a plant from the ground up, whereas H2SO4 properly inhibited the fumes are local around the acid tank and not nearly so corrosive to the entire building and equipment.
Regards,

Dr. Thomas H. Cook, Galvanizing Consultant Hot Springs, South Dakota, USA

Use of Sulfuric acid is working fine and better than HCl. What should be the suitable Effluent treatment for used Sulfuric acid in this case?

Mahesh Behra
Surfactant Solutions - Pune, India